This invention relates to opaque photographic papers suitable as a base assembly for the production of negative papers for use in self-developing film packs for diffusion type photographic processes.
In diffusion type photographic processes wherein the photographic print is obtained by pod development, outside of the camera, of the negative in contact with the positive print paper, it is essential that the negative as well as the positive print paper be impervious to actinic light. For purposes of this invention, actinic light is any radiation which causes photochemical action, e.g., visible light. Opaque papers for this use must be substantially free of pinholes.
U.S. Pat. No. 3,411,908 (1968) to Crawford et al. and U.S. Pat. No. 3,753,764 (1973) to Haefner each suggest the use of carbon black in a single backcoating of polyethylene for opacity. U.S. Pat. No. 3,531,514 (1970) to Kerr et al. and U.S. Pat. No. 3,582,339 (1971) to Martens et al. each suggest the use of carbon black as an internal antistat in a backcoating of polyethylene.
We have found that the degree of opacity necessary for use as a negative of a self-developing film pack such as a "Polaroid" pack cannot be obtained with a single extruded carbon black pigmented polyethylene layer. Polaroid is a trademark of Polaroid Corporation, Cambridge, Mass. A single layer of carbon black pigmented polyethylene does not appear to be sufficient due to a very small amount of "grit" which is normally present in the carbon black as commercially manufactured. These grit particles are small clear silicate particles which cause clear unpigmented spots in the polyethylene layer resulting in optical pinholes under actinic light. The optical pinholes appear as a black spot in the negative emulsion after photographic development. A sufficient number of these grit particles are apparently large enough that even at greatly increased coating thicknesses opacity failure due to pinholes cannot be entirely eliminated using only one polyethylene layer containing carbon black.
U.S. Pat. No. 3,615,550 (1971) to Kemme and U.S. Defensive Publication No. T875,014 (Published 1970) to Campbell et al. each disclose that a single layer of polyolefins, e.g., polyethylene, containing carbon black is not sufficient to produce an opaque paper. Each of these disclosures applies an aqueous coating containing carbon black as the principal pigment of a backside coating for opacity. Campbell et al. also adheres a polyolefin layer containing carbon black to the aqueous applied carbon black layer, and Kemme preferably adds carbon black to his polyolefin sealing coat. Campbell et al. apply their aqueous coating directly to the surface of the paper thereby subjecting the paper to swelling during application of the coating and contraction of the paper during drying thereby setting up stresses in the resultant paper. Both Campbell et al. and Kemme require equipment for applying and drying aqueous coatings in addition to extrusion equipment for applying the polyethylene layers.
Commonly-assigned U.S. Pat. No. 3,758,376 (1973) to Beckner et al. discloses the use of light absorbing colorants, e.g., carbon black, in combination with light scattering white pigments in the paper layer to produce a gray opaque paper having a thickness (caliper) of about 7.5 mils. Such a gray opaque paper may be used as a base for positive print papers in pod development in diffusion type photographic processes. The thickness of the paper is desirable to give stiffness and body to the resultant printed picture.
For negative papers, it is desirable to reduce the caliper of the paper layer, and thus the overall caliper of the negative, as much as possible so that the film pack may be kept at a reasonable thickness. At these low calipers which may be as low as 2 mils, loading of these papers with titanium dioxide and carbon black, as taught by Beckner et al., would severely reduce the strength of the paper layer.
We have found that many of the difficulties experienced in the prior art may be eliminated by coating both sides of a layer of paper with a layer of polyolefin. If two out of three of the above layers contain carbon black, a photographic film assembly substantially free of optical pinholes can be produced since alignment of the grit particles in the two layers containing carbon black is practically not possible. Thus, a more efficient use of carbon black can be realized.